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Similar Hormone-Rich Peptides from Thyroglobulins of Five Vertebrate Classes^*

Divisions of Endocrinology and General Medicine, Department of Medicine, University of Virginia School of Medicine Charlottesville, Virginia 22908

Address reprint request to: Dr. J. T. Dunn, Box 511, University of Virginia School of Medicine, Charlottesville, Virginia 22908.

Abstract

Thyroglobulins (Tgs) were purified from five species (rat, chicken, turtle, frog, and goldfish), each representing a different vertebrate class. On reduction with mercaptoethanol, each Tg produced five major iodopeptides, designated A–E, with ranges of estimated molecular mass, in kilodaltons (K), as follows: A, more than 300K; B, 210–280K; C, 30–42K; D, 19–28K; and E, 10–23K. Of these, the two smallest, D and E, had 40–80% of their iodine as iodothyronine, compared with 15–20% for the parent Tgs. They contained 25–63% of Tg’s total iodothyronines but only a few percent of its peptide material. Calculations from amino acid analyses and iodine contents showed approximately 1 mol each of D and E/mol 660,000 dalton Tg. In comparisons of amino acid compositions by cluster analysis, iodopeptides D and E resembled each other and their counterparts in other species more than they resembled their parent Tgs. Also, the Tgs from different species were more similar to each other and to iodopeptides D and E than to nonthyroidal proteins randomly selected from the literature.

We injected ¹²⁵I into rats and turtles, and compared its distribution among the iodopeptides to that of ¹²⁷I. These dual isotope experiments showed that as Tg was iodinated in vivo, iodopeptide B decreased both in molecular size and in its share of Tg’s iodine, while the sum of iodopeptides D and E increased, indicating that B may be the precursor of D and E. In vivo iodination of rat Tg with ¹²⁵I for different periods of time suggested that iodopeptide E and its iodothyronines are derived from a larger portion of the Tg molecule, perhaps iodopeptide B. The amount of ¹²⁵I in iodopeptide D also increased with time.

We conclude that these two discrete hormone-rich peptide fractions of small molecular weight are a consistent feature of Tg’s structure throughout the vertebrate classes. Both are probably derived from a larger iodopeptide fraction (iodopeptide B) during the course of Tg maturation and iodination. Their conservation during evolution indicates an important role for them in thyroid hormone biosynthesis. (Endocrinology 114: 369, 1984)

Footnotes

^* Supported in part by Grants AM-11043 and 5-T35-AM-0719 from the NIH.

Received May 19, 1983.